A mailer is a container designed to protect something while it's being sent through the mail. When you order a book online, it often arrives in a padded mailer: a thick envelope with bubble wrap inside that cushions the book during shipping. Companies use mailers to send everything from documents to small electronics, choosing different types depending on what needs protection.

The word can also refer to an advertisement or announcement sent through the mail. Political candidates send out mailers before elections, explaining their positions and asking for votes. Stores send mailers with coupons and special offers. These printed pieces are designed to grab your attention when you open your mailbox.

Less commonly, a mailer can mean a person or company whose job is sending things through the mail, though this usage is rare today.

The key idea behind all these meanings is the same: a mailer is something that helps get information or objects from one place to another through the postal system. Whether it's protecting a fragile item or delivering a message to hundreds of homes, mailers are tools for shipping and communication.